Liquid crystal display devices are mounted at various devices, for example, a television receiver, a vehicle-mounted display such as a car navigation device, and mobile terminals such as a laptop computer, a tablet personal computer (PC), a cell phone, and a smart phone.
For example, in a twisted nematic (TN) mode liquid crystal display device and an optically compensated bend (OCB) mode liquid crystal display device, an alignment direction of liquid crystal molecules included in a liquid crystal layer held between an upper substrate and a lower substrate is controlled by a vertical electric field generated between the common electrode and the pixel electrodes.
In an in-plane switching (IPS) mode liquid crystal display device and a fringe-field switching (FFS) mode liquid crystal display device, a common electrode and pixel electrodes are provided in the same substrate, and an alignment direction of liquid crystal display molecules included in a liquid crystal layer is controlled by an electric field (fringe electric field) generated between the common electrode and the pixel electrodes. The FFS mode liquid crystal display device can ensure a great aperture ratio, and thus has a high brightness and a good viewing angle characteristic.
In particular, in liquid crystal display devices for use in mobile terminals such as a smart phone, it is indispensable that the power consumption of a circuit is reduced. Thus, as to matters related to the power consumption of a circuit, e.g., a polarity inversion type, it is necessary to select an optimal type. Generally, frame-inversion, line-inversion, column-inversion, dot-inversion, etc. are well known as polarity inversion types. In the line-inversion and the dot-inversion, writing is performed on pixels, while inverting the polarity in units of one line at the time of scanning a screen, as a result of which it is therefore necessary to perform charging and discharging of source lines at intervals of 1 horizontal (H) period, and the power consumption is increased.
Furthermore, in the frame inversion, the power consumption of a circuit is small; however, since the entire screen have the same polarity, if positive and negative luminance responses are made as different margins due to charging-up (image burn-in), etc., a flicker easily occurs.
In another aspect, a low-frequency drive scheme and an intermittent driving are proposed as means for reducing the power consumption of a circuit. The low-frequency drive scheme is a scheme to reduce power of a circuit by lowering a drive frequency of a liquid crystal display device to, e.g., ½ or ¼ of that under standard conditions. The intermittent driving is a scheme to reduce power of a circuit by stopping the circuit for a time period corresponding to several frames after wiring is performed for a single display time period (a single frame) of a liquid crystal display device. In both those drive schemes, since a frequency of rewriting a video signal in the liquid crystal display unit is small, for example, a blur can be generated in moving images. However, the drive schemes are effective measures to reduce power of a circuit in the case where they are applied to displaying of a still image or the like whose visibility is held in little account.
It should be noted that in both the above drive schemes, when the frequency of rewriting the video signal in the liquid crystal display unit is lowered, a polarity inversion frequency of a voltage to be applied to liquid crystal is inevitably lowered.